Interest in flat-panel loudspeaker technologies has seen a marked increase in the last 10 years. Essentially, this is due to the increased space requirements of modern sound reproduction methods such as 5.1 surround or wave field synthesis, and to the diminishing installation space for loudspeakers in increasingly small and/or flat multimedia devices such as mobile phones and notebooks, for example. Utilization of flat-panel loudspeakers rather than conventional loudspeakers is to meet said increased requirements.
Investigations made on various flat-panel speaker technologies, which typically are as old as the cone loudspeakers by Kellogg and Rice, have shown that both utilization of non-housed flat-panel speakers directly on the wall and utilization of a flat loudspeaker housing entail considerable losses of sonic quality. Conventional technology may be found in Beer, D.: Flachlautsprecher—ein Überblick [Flat-panel loudspeakers—an Overview], presented at the DAGA08 trade fair, March 2008, Dresden; H. Azima, J. Panzer, “Distributed-Mode Loudspeakers (DML) in Small Enclosures”, presented at the 106th AES Convention, Munich, Germany, May 1999; Beer et al.: The air spring effect of flat panel speakers, presented at the 124th AES Convention, May 2008, Amsterdam/The Netherlands; and Wagner, Roland: Electrostatic Loudspeaker—Design and Construction. Audio Amateure Press, Peterborough, N.H., 1993.
A non-housed flat-panel speaker typically is a dipole radiator having a low sound pressure level in the low-frequency tone range due to the acoustic short circuit. When such a dipole is installed near a wall, reflection and superposition of the rearward sound component with the portions of the sound that is emitted on the front side of the diaphragm, and diffraction effects associated therewith will lead to comb-filter-type sound coloration above the short-circuit frequency. It is for this reason that for conventional loudspeakers, loudspeaker housings are used. However, to preserve the advantage of a flat design, one uses flat housings that typically enclose a relatively small air volume. Just like with conventional speakers, too small an air volume will raise the fundamental resonant frequency of the sound transducer. Consequently, the lower cutoff frequency will also rise, which will result in reduced low-frequency tone reproduction.
US 2005/0201583 A1 discloses a low-frequency two-dimensional array based on a dipole principle. The system includes a support system having an open frame, several sub-woofers being accommodated in the open frame system in a dipole two-dimensional array configuration so as to provide controlled sound dispersion both in the horizontal and vertical planes. The sub-woofers are operable to provide low-frequency sound dispersion below about 300 Hz.
DE 695 07 896 T2 discloses a speaker device having controlled directional sensitivity and having a first set of at least three speakers arranged along a first straight line in accordance with a predetermined pattern, the distances from speaker to speaker being configured in a variable manner, and it also being possible for speakers to be arranged such that they are in contact with one another.
U.S. Pat. No. 2,602,860 discloses a speaker structure wherein nine conical speakers are symmetrically arranged, within one single frame, in three rows of three, respectively. The frame includes mutually tilted segments to increase the angle of radiation. For example, the distance between the edges of the speakers is to be smaller than the radius of the speakers, all of the speakers being operated from one same source. In addition, no restriction regarding movement of air is to be achieved by a housing, since this would adversely affect the performance at low frequencies.
U.S. Pat. No. 4,399,328 discloses a column, which is independent of direction and frequency, of electroacoustic transducers controlled using different amplitudes, so that specific conditions of the control operation of the electroacoustic transducers will result.
U.S. Pat. No. 6,801,631 B1 discloses a speaker system featuring several transducers positioned within a plane to achieve an optimum acoustic sound radiation pattern. Four central transducers (woofers) cooperate to reproduce the low and medium frequencies, the woofers being positioned such that no two woofers share a common vertical axis or a common horizontal axis. In addition, a fifth transducer, specifically a high-frequency tweeter, is provided which is arranged at a central location in between the woofers.